convert vector to list

Question

Hi All,

How to convert a vector to a list?

Answer 1

Vector is a concrete class that implements the List interface so technically it is already a List. You can do this:

List list = new Vector();

or:

List<String> list = new Vector<String>();

(assuming a Vector of Strings).

If however you want to convert it to an ArrayList, which is the closest List implementation to a `Vector~ in the Java Collections Framework then just do this:

List newList = new ArrayList(vector);

or for a generic version, assuming a Vector of Strings:

List<String> newList = new ArrayList<String>(vector);

Answer 2

If you want a utility method that converts an generic Vector type to an appropriate ArrayList, you could use the following:

public static <T> ArrayList<T> toList(Vector<T> source) {
    return new ArrayList<T>(source);
}

In your code, you would use the utility method as follows:

public void myCode() {
    List<String> items = toList(someVector);
    System.out.println("items => " + items);
}

You can also use the built-in java.util.Collections.list(Enumeration) as follows:

public void myMethod() {
    Vector<String> stringVector = new Vector<String>();
    List<String> theList = Collections.list(stringVector.elements());
    System.out.println("theList => " + theList);
}

But like someone mentioned below, a Vector is-a List! So why would you need to do this? Perhaps you don't want some code you use to know it's working with a Vector - perhaps it is inappropriately down-casting and you wish to eliminate this code-smell. You could then use

// the method i give my Vector to can't cast it to Vector
methodThatUsesList( Collections.unmodifiableList(theVector) );

if the List should be modified. An off-the-cuff mutable wrapper is:

public static <T> List<T> asList(final List<T> vector) {
    return new AbstractList<T>() {
        public E get(int index) { return vector.get(index); }
        public int size() { return vector.size(); }
        public E set(int index, E element) { return vector.set(index, element); }
        public void add(int index, E element) { vector.add(index, element); }
        public E remove(int index) { return vector.remove(index); }
    }
}